Tasshi Dennis and collaborators at NIST and NREL in Boulder, CO, have developed a powerful new research tool to arbitrarily shape the spectral output of a SuperK supercontinuum laser.
The tool allows the user to request an arbitrary spectrum, like an AM 1.5 solar reference spectrum, or a flat or Gaussian shaped spectrum, via computer control. It also lets the researchers at NIST probe the performance of photovoltaic (PV) materials as a function of the solar spectrum .
The spatial coherence of the white-light laser allows the simulation of solar concentrator power levels to over 600 suns [2,3], and to make 2D efficiency maps of sample devices .
These efforts have been focused mainly on multi-junction cells with broadband absorption characteristics to maximize the efficiency of PV cells.
Features and benefits
Supercontinuum lasers generate a broad spectrum from under 400 nm to over 2500 nm. The spectrum is dictated by the fiber and the peak power in the fiber, due to the nonlinear optical processes that generate it. The complex physics involved causes it to be impractical to generate a specific spectral shape as many applications researchers would benefit from.
Several tunable filters are currently offered to select certain lines or bandwidths, but researchers at NIST set out to arbitrarily shape a spectrum for purposes of multi-junction photovoltaics and solar concentrator experimental work.
For these applications, it is of interest to simulate a typical solar spectrum, but even within the solar spectrum, there are many variations of spectral shape and power. The actual solar spectrum varies with the seasons, the hour of the day, and atmospheric conditions, hence the need to test devices under several different light spectrum conditions .
The researchers at NIST came up with an experimental system shown in Figure 1. A supercontinuum laser beam is split into two spectral regions. Each beam travels through similar devices that separate the spectrum in one dimension and apply tunable reflective devices (spatial light modulator [LCOS] and grating light valve [GLV] ). The post-processed beams are then recombined and injected into a broadband fiber for delivery to a sample or to a spectrometer.
Figure 1. Experimental setup for the spectral shaper. LCOS is a liquid crystal on a silicon spatial light modulator and GLV is a grating light valve from silicon light machines.
A spectral shaping filter like this is capable of transforming a standard supercontinuum laser spectrum to match virtually any application requirements.
In this case, it allows researchers to request – at the press of a button – a solar spectrum to match a summer day, a winter day, a cloudy day, different times of the day, and many other conditions. A few specific examples of the achievable spectra are shown in Figure 2.
Figure 2. Shaped supercontinuum spectra to match solar spectrum for the time of day or atmospheric condition reference spectra
Beyond shaping the spectrum, the intrinsic spatial coherence of the supercontinuum source allows for the shaped spectrum to be focused to diffraction-limited spot size. This, in turn, can provide the optical power density of 100’s of suns onto a PV sample, which is key or solar concentrator research.
The spatial coherence also provides the capability to provide 2D efficiency/performance maps of PV cells as shown in Figure 3, which is not easily achievable with other lamp-based illumination sources .
Figure 3. 2D images of a PV cell current production showing (a) a defect on the cell and (b) non-producing wires on the cell.